The present invention relates to a method for the vapor-phase carbonylation of lower alkyl alcohols, ether and ester derivatives of the alcohols, and mixtures thereof to produce esters and carboxylic acids. Particularly, the present invention relates to a method for the vapor-phase carbonylation of methanol to produce acetic acid and methyl acetate using a solid catalyst having platinum and tin associated with a solid support material.
Lower carboxylic acids and esters such as acetic acid and methyl acetate have been known as industrial chemicals for many years. Acetic acid is used in the manufacture of a variety of intermediary and end-products. For example, an important derivative is vinyl acetate that can be used as a monomer or co-monomer for a variety of polymers. Acetic acid itself is used as a solvent in the production of terephthalic acid, which is widely used in the container industry, and particularly in the formation of PET beverage containers.
There has been considerable research activity in the use of metal catalysts for the carbonylation of lower alkyl alcohols, such as methanol, and ethers to their corresponding carboxylic acids and esters, as illustrated in equations 1-3 below:
ROH+COxe2x86x92RCOOHxe2x80x83xe2x80x83(1)
2ROH+COxe2x86x92RCOOR+waterxe2x80x83xe2x80x83(2)
RORxe2x80x2+COxe2x86x92RCOORxe2x80x83xe2x80x83(3)
Carbonylation of methanol is a well-known reaction and is typically carried out in the liquid phase with a catalyst. A thorough review of these commercial processes and other approaches to accomplishing the formation of acetyl from a single carbon source is described by Howard et al. in Catalysis Today, 18, 325-354 (1993). Generally, the liquid phase carbonylation reactions for the preparation of acetic acid using methanol are performed using homogeneous catalyst systems comprising a Group VIII metal and a halogen component such as iodine or bromine or an iodine or bromine-containing compound such as hydrogen iodide, hydrogen bromide, methyl iodide, or methyl bromide. Rhodium is the most common Group VIII metal catalyst and methyl iodide is the most common promoter. These reactions are conducted in the presence of water to prevent precipitation of the catalyst.
These recently developed processes represent a distinct improvement over the classic carbonylation processes wherein such feed materials have been previously carbonylated in the presence of such catalyst systems as phosphoric acid, phosphates, activated carbon, heavy metal salts and metal carbonyls such as cobalt carbonyl, iron carbonyl and nickel carbonyl. All of these previously known processes require the use of extremely high partial pressures of carbon monoxide. They also have the disadvantage of requiring higher catalyst concentrations, longer reaction times, and higher temperatures to obtain substantial reaction and conversion rates. This results in needing larger and more costly processing equipment and higher manufacturing costs.
A disadvantage of a homogeneous phase carbonylation process is that additional steps are necessary for separating the products from the catalyst solutions, and there are always handling losses of the catalyst. Losses of the metal in the catalyst can be attributed to several factors, such as the plating-out of the active metal onto piping and process equipment thereby rendering the metal inactive for carbonylation purposes and losses due to incomplete separation of the catalyst from the products. These losses of the metal component are costly because the metals themselves are very expensive.
Schultz, in U.S. Pat. No. 3,689,533, discloses using a supported rhodium heterogeneous catalyst for the carbonylation of alcohols to form carboxylic acids in a vapor-phase reaction. Schultz further discloses the presence of a halide promoter.
Schultz in U.S. Pat. No. 3,717,670 describes a similar supported rhodium catalyst in combination with promoters selected from Groups IB, IIIB, IVB, VB, VIB, VIII, lanthanide and actinide elements of the Periodic Table.
Uhm, in U.S. Pat. No. 5,488,143, describes the use of alkali, alkaline earth or transition metals as promoters for supported rhodium for the halide-promoted, vapor phase methanol carbonylation reaction. Pimblett, in U.S. Pat. No. 5,258,549, teaches that the combination of rhodium and nickel on a carbon support is more active than either metal by itself.
In addition to the use of iridium as a homogeneous alcohol carbonylation catalyst, Paulik et al., in U.S. Pat. No. 3,772,380, describe the use of iridium on an inert support as a catalyst in the vapor phase, halogen-promoted, heterogeneous alcohol carbonylation process.
European Patent Application EP 0 759 419 A1 pertains to a process for the carbonylation of an alcohol and/or a reactive derivative thereof. EP 0 759 419 A1 discloses a carbonylation process comprising a first carbonylation reactor wherein an alcohol is carbonylated in the liquid phase in the presence of a homogeneous catalyst system and the off gas from this first reactor is then mixed with additional alcohol and fed to a second reactor containing a supported catalyst. The homogeneous catalyst system utilized in the first reactor comprises a halogen component and a Group VIII metal selected from rhodium and iridium. When the Group VIII metal is iridium, the homogeneous catalyst system also may contain an optional co-promoter selected from the group consisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc, indium and gallium. The supported catalyst employed in the second reactor comprises a Group VIII metal selected from the group consisting of iridium, rhodium, and nickel, and an optional metal promoter on a carbon support. The optional metal promoter may be iron, nickel, lithium and cobalt. The conditions within the second carbonylation reactor zone are such that mixed vapor and liquid phases are present in the second reactor. The presence of a liquid phase component in the second reactor inevitably leads to leaching of the active metals from the supported catalyst which, in turn, results in a substantial decrease in the activity of the catalyst and costly replacement of the active catalyst component.
The literature contains several reports of the use of rhodium-containing zeolites as vapor phase alcohol carbonylation catalysts at one bar pressure in the presence of halide promoters. The lead references on this type of catalyst are presented by Maneck et al. in Catalysis Today, 3, 421-429 (1988). Gelin et al., in Pure and Appl. Chem., Vol. 60, No. 8, 1315-1320 (1988), provide examples of the use of rhodium or iridium contained in zeolite as catalysts for the vapor phase carbonylation of methanol in the presence of halide promoter. Krzywicki et al., in Journal of Molecular Catalysis, 6, 431-440 (1979), describe the use of silica, alumina, silica-alumina and titanium dioxide as supports for rhodium in the halide-promoted vapor phase carbonylation of methanol, but these supports are generally not as efficient as carbon. Luft et al., in U.S. Pat. No. 4,776,987 and in related disclosures, describe the use of chelating ligands chemically attached to various supports as a means to attach Group VIII metals to a heterogeneous catalyst for the halide-promoted vapor phase carbonylation of ethers or esters to carboxylic anhydrides.
Evans et al., in U.S. Pat. No. 5,185,462, describe heterogeneous catalysts for halide-promoted vapor phase methanol carbonylation based on noble metals attached to nitrogen or phosphorus ligands attached to an oxide support.
Panster et al., in U.S. Pat. No. 4,845,163, describe the use of rhodium-containing organopolysiloxane-ammonium compounds as heterogeneous catalysts for the halide-promoted liquid phase carbonylation of alcohols.
Drago et al., in U.S. Pat. No. 4,417,077, describe the use of anion exchange resins bonded to anionic forms of a single transition metal as catalysts for a number of carbonylation reactions including the halide-promoted carbonylation of methanol. Although supported ligands and anion exchange resins may be of some use for immobilizing metals in liquid phase carbonylation reactions, in general, the use of supported ligands and anion exchange resins offer no advantage in the vapor phase carbonylation of alcohols compared to the use of the carbon as a support for the active metal component. Typically, these catalysts are unstable at elevated temperatures making them poorly suited to vapor phase processes.
Nickel on activated carbon has been studied as a heterogeneous catalyst for the halide-promoted vapor phase carbonylation of methanol. Relevant references to the nickel-on-carbon catalyst systems are provided by Fujimoto et al. in Chemistry Letters 895-898, (1987). Moreover, Fujimoto et al. in Journal of Catalysis, 133, 370-382 (1992) observed increased rates when hydrogen is added to the feed mixture. Liu et al., in Ind. Eng. Chem. Res., 33 488-492, (1994), report that tin enhances the activity of the nickel-on-carbon catalyst. Mueller et al., in U.S. Pat. No. 4,918,218, disclose the addition of palladium and optionally copper to supported nickel catalysts for the halide-promoted carbonylation of methanol. In general the rates of reaction provided by nickel-based catalysts are lower than those provided by the analogous rhodium-based catalysts when operated under similar conditions.
Other single metals supported on carbon have been reported by Fujimoto et al. in Catalysis Letters, 2, 145-148 (1989) to have limited activity in the halide-promoted vapor phase carbonylation of methanol. The most active of these metals is Sn. Following Sn in order of decreasing activity are Pb, Mn, Mo, Cu, Cd, Cr, Re, V, Se, W, Ge and Ga. None of these other single metal catalysts are nearly as active as those based on Rh, Ir, Ni or the catalyst of the present invention.
Yagita and Fujimoto in Journal of Molecular Catalysis, 69, 191-197 (1991) examined the role of activated carbon in a metal supported catalyst and observed that the carbonylation activities of Group VIII metals supported on activated carbon are ordered by the affinities between the metal and the halogen.
Feitler, in U.S. Pat. No. 4,612,387, describes the use of certain zeolites containing no transition metals as catalysts for the halide-free carbonylation of alcohols and other compounds in the vapor phase.
U.S. Pat. No. 5,218,140, describes a vapor phase process for converting alcohols and ethers to carboxylic acids and esters by the carbonylation of alcohols and ethers with carbon monoxide in the presence of a metal ion exchanged heteropoly acid supported on an inert support. The catalyst used in the reaction includes a polyoxometallate anion in which the metal is at least one of a Group V(a) and VI(a) is complexed with at least one Group VIII cation such as Fe, Ru, Os, Co, Rh, Ir, Ni, Pd or Pt as catalysts for the halide-free carbonylation of alcohols and other compounds in the vapor phase.
In accordance with the present invention, a method for the heterogeneous vapor-phase carbonylation of reactants comprising lower alkyl alcohols, ether and ester derivatives of the alcohols, and ester-alcohols mixtures is provided using a catalyst that includes a catalytically effective amount of platinum and tin associated with a solid support material which, desirably, is inert to the carbonylation reaction.
Briefly, the present invention is a method for the vapor-phase carbonylation of reactants comprising lower alkyl alcohols, lower alkyl alcohol generating compositions such as ether and ester derivatives of the alcohols, and mixtures thereof for producing esters and carboxylic acids. The method includes contacting the reactants under vapor-phase carbonylation reaction conditions with a heterogeneous catalyst having a catalytically effective amount of platinum and/or platinum salt and tin and/or tin salt associated with a solid support material. In a preferred embodiment, the method also includes contacting the reactants, in the presence of the solid catalyst, with a vaporous halide promoter. As used herein the term xe2x80x9cassociated withxe2x80x9d means any manner for incorporating or associating the platinum metal and/or its salt and the tin metal and/or its salt on or in the solid support material. Non-limiting examples in which the platinum and tin metals or their respective salts may be associated with the solid support include impregnating, immersing, spraying and coating the support with a solution containing platinum and with a solution containing tin sequentially or impregnating, immersing, spraying and coating the support with a solution containing a mixture of platinum and tin.
It is an object of the invention to provide a method for the carbonylation of lower alkyl alcohols, ethers, and ester-alcohol mixtures to produce esters and carboxylic acids. More particularly, it is an object of the present invention to provide a vapor-phase carbonylation method for the production of acetic acid, methyl acetate and mixtures thereof from a lower alkyl alcohol and preferably, methanol.
It is another object of the invention to provide a process in which the catalyst is maintained in a solid phase to reduce or eliminate the handling losses of the catalyst.
It is another object of the invention to provide a vapor phase carbonylation process for the production of acetic acid and methyl acetate which utilizes a more stable catalyst and reduces the need for catalyst recovery and recycle as well as solvent recovery.
These and other objects and advantages of the invention will become apparent to those skilled in the art from the accompanying detailed description.
In accordance with the invention, a vapor-phase carbonylation method is provided for the continuous production of carboxylic acids and esters by contacting lower alkyl alcohols, lower alkyl alcohol generating compositions such as ether and/or ester derivatives of the alcohol, and mixtures thereof, and carbon monoxide with a solid supported catalyst. The catalyst includes an effective amount of platinum and/or platinum salt and tin and/or tin salt associated with a solid support material which, desirably, is inert to the carbonylation reaction. In a preferred embodiment of the present process, the reactant is fed in conjunction with a vaporous halide promoter. In a preferred embodiment, the present invention provides for the vapor-phase carbonylation of methanol for the continuous production of acetic acid, methyl acetate or mixtures thereof.
The process of this invention is operated in the vapor phase and, therefore, is practiced at temperatures above the dew point of the reactants and product mixture, i.e., the temperature at which condensation occurs. However, since the dew point is a complex function of dilution (particularly with respect to non-condensable gases such as unreacted carbon monoxide, hydrogen, or inert diluent gas), reactant and product composition, and pressure, the process may still be operated over a wide range of temperatures, provided the temperature exceeds the dew point of the reactants and products. In practice, this generally dictates a temperature range of about 100xc2x0 C. to 500xc2x0 C., with temperatures in the range of 100xc2x0 C. to 325xc2x0 C. being preferred and temperature of about 150xc2x0 C. to 275xc2x0 C. being particularly useful. Advantageously, operating in the vapor phase eliminates catalyst dissolution, i.e., metal leaching from the catalyst support, which occurs in the known heterogeneous processes operating in the presence of liquid compounds.
As with temperature, the useful pressure range for the vapor phase carbonylation is limited by the dew point of the product mixture. Provided that the reaction is operated at a temperature sufficient to prevent liquefaction of the reactants and products, a wide range of pressures may be used, e.g., pressures in the range of about 0.1 to 100 bars absolute. The process preferably is carried out at a pressure in the range of about 1 to 50 bars absolute, most preferably, about 3 to 30 bar absolute (bara).
Suitable feedstock, i.e., reactants, for carbonylation using the catalyst of the present invention include lower alkyl alcohols, lower alkyl alcohol generating compositions such as ether and ester derivatives of the alcohols, and mixtures thereof. Non-limiting examples of reactants include alcohols and ethers in which an aliphatic carbon atom is directly bonded to an oxygen atom of either an alcoholic hydroxyl group in the compound or an ether oxygen in the compound and may further include aromatic moieties. Preferably, the feedstock is one or more lower alkyl alcohols having from 1 to 10 carbon atoms and preferably having from 1 to 6 carbon atoms, alkane polyols having 2 to 6 carbon atoms, alkyl alkylene polyethers having 3 to 20 carbon atoms and alkoxyalkanols having from 3 to 10 carbon atoms. The most preferred reactant is methanol. Although methanol is preferably used in the process and is normally fed as methanol, it can be supplied in the form of a combination of materials which generate methanol. Examples of such combination of materials include (i) methyl acetate and water and (ii) dimethyl ether and water. In the operation of the process, both methyl acetate and dimethyl ether are formed within the reactor and, unless methyl acetate is the desired product, they are recycled with water to the reactor where they are later consumed to form acetic acid.
Although the presence of water in the gaseous feed mixture is not essential when using methanol, the presence of some water is desirable to suppress formation of methyl acetate and/or dimethyl ether. When using methanol to generate acetic acid, the molar ratio of water to methanol can be 0:1 to 10:1, but preferably is in the range of 0.01:1 to 1:1. When using an alternative source of methanol such as methyl acetate or dimethyl ether, the amount of water fed usually is increased to account for the mole of water required for hydrolysis of the methanol alternative. Accordingly, when using either methyl acetate or dimethyl ether, the mole ratio of water to ester or ether is in the range of 1:1 to 10:1, but preferably in the range of 1:1 to 3:1. In the preparation of acetic acid, it is apparent that combinations of methanol, methyl ester, and/or dimethyl ether are equivalent, provided the appropriate amount of water is added to hydrolyze the ether or ester to provide the methanol reactant.
When the methyl ester, methyl acetate, is the desired product, no water should be added to the carbonylation process and dimethyl ether becomes the preferred feedstock. Further, when methanol is used as the feedstock in the preparation of methyl acetate, it is necessary to remove water. However, the primary utility of the process of the present invention is in the manufacture of acetic acid.
In the practice of a vapor-phase carbonylation process, the reactant, in the vapor phase, is passed through or over the solid supported platinum and tin catalyst and preferably the reactant is fed in conjunction with a vaporous halide promoter.
In preparing the solid supported catalyst, the form of platinum used generally is not critical. The solid supported component of the catalyst may be prepared from a wide variety of platinum containing compounds. For example, the platinum compound can be in the form of a salt of a mineral acid halide, such as chloroplatinic acid; trivalent nitrogen compounds such as dichlorodiammine platinum; organic compounds of trivalent phosphorous, such as dichlorobis(triphenylphosphine) platinum; olefins, such as dichloro(1,5-cyclooctadiene) platinum; nitrites, such as dichlorobis(benzonitrile) platinum and oxides of platinum may be used if dissolved in the appropriate medium either alone or in combination. The preferred sources of platinum is one of it chlorides, such as any of the various salts of hexachloroplatinate(IV) or a solution of platinum dichloride in either aqueous HCl or aqueous ammonia.
The amount of platinum, as metal, on the support can vary from about 0.01 weight percent to about 10 weight percent, with from about 0.1 weight percent to about 2 weight percent platinum being preferred, wherein the weight percent is based on the total weight of the solid catalyst.
Likewise, the form of tin used in preparing the solid supported catalyst is not critical and may include a wide variety of tin containing compounds. For example, suitable tin compounds which may be associated with the solid support material include tin halides such as tin (II) chloride; alkyl carboxylate salts and aryl carboxylate salts wherein the alkyl group has from 1 to 10 carbon atoms and the aryl group has from 6 to 24 carbon atoms wherein at least one of the carbon atoms is bound to the tin moiety, tin oxides such as tin (II) oxalate, and mixtures of such tin containing compounds. The preferred sources of tin are tin (II) chloride, preferably dissolved in aqueous HCl, and tin (II) oxalate due to their availability, cost, lower toxicity, and high solubility in water (the preferred solvent medium).
The content of tin, as metal, on the support can vary over a wide range, for example from about 0.01 to 10 weight percent tin based on the total weight of the solid supported catalyst. However, the preferred amount of tin in the catalyst is from about 0.1 to 5 weight percent of tin based on the total weight of the solid supported catalyst.
The solid support useful for acting as a carrier for the platinum and tin consists of a porous solid of such size that it can be employed in fixed or fluidized bed reactors. Typical support materials have a size of from about 400 mesh per inch to about xc2xd inch. Preferably, the support is carbon, including activated carbon, having a high surface area. Activated carbon is well known in the art and may be derived from coal or peat having a density of from about 0.03 grams/cubic centimeter (g/cm3) to about 2.25 g/cm3. The carbon can have a surface area of from about 200 square meters/gram (m2/g) to about 1200 m2/g. Other solid support materials which may be used in accordance with the present invention include pumice, alumina, silica, silica-alumina, magnesia, diatomaceous earth, bauxite, titania, zirconia, clays, magnesium silicate, silicon carbide, zeolites, and ceramics. The shape of the solid support is not particularly important and can be regular or irregular and include extrudates, rods, balls, broken pieces and the like disposed within the reactor.
The platinum and tin can be associated with the solid support by solubilizing the metals, or their respective salts, in a suitable solvent and contacting the solubilized platinum and tin with the solid support material. The solvent is then evaporated so that at least a portion of the platinum and tin is associated with the solid support. Drying temperatures can range from about 100xc2x0 C. to about 600xc2x0 C. for a period greater than about one second. One skilled in the art will understand that the drying time is dependent upon the temperature, humidity, and solvent. Generally, lower temperatures require longer heating periods to effectively evaporate the solvent from the solid support. The method of preparing the solid component of the catalyst optionally further includes the step of heating the solid supported platinum and tin in a stream of inert gas. Non-limiting examples of suitable inert gases include nitrogen, argon and helium
Alternatively the platinum and tin can be associated with the solid support by sequentially associating each metal with a support material. For example, platinum or a platinum containing salt would be solubilized using a suitable solvent. The dissolved metal solution would then be contacted with the support material. Afterwards, the solvent is evaporated so that at least a portion of the platinum is associated with the solid support material. Next, tin or a tin containing salt would be associated with the support material following a similar procedure as described for associating the platinum with the solid carrier. Thus, one will understand that multiple layers of the respective platinum and tin metals or metal containing compounds can be associated with the support by merely following multiple steps of the procedure described above.
In a preferred embodiment, the method further includes contacting the reactants, in the presence of the solid catalyst, with a vaporous halide promoter selected from chlorine, bromine and iodine compounds. Preferably, the vaporous halide is selected from bromine and iodine compounds that are vaporous under vapor-phase carbonylation conditions of temperature and pressure. Suitable halides include hydrogen halides such as hydrogen iodide and gaseous hydriodic acid; alkyl and aryl halides having up to 12 carbon atoms such as, methyl iodide, ethyl iodide, 1-iodopropane, 2-iodobutane, 1-iodobutane, methyl bromide, ethyl bromide, benzyl iodide and mixtures thereof. Desirably, the halide is a hydrogen halide or an alkyl halide having up to 6 carbon atoms. Non-limiting examples of preferred halides include hydrogen iodide, methyl iodide, hydrogen bromide, methyl bromide and mixtures thereof The halide may also be a molecular halide such as I2, Br2, or Cl2.
The molar ratio of methanol or methanol equivalents to halide present to produce an effective carbonylation ranges from about 1:1 to 10,000:1, with the preferred range being from about 5:1 to about 1000:1.
In a preferred aspect of the invention, the vapor-phase carbonylation method of the present invention may be used for making acetic acid, methyl acetate or a mixture thereof The process includes the steps of contacting a gaseous mixture comprising methanol and carbon monoxide with a solid supported catalyst in a carbonylation zone and under carbonylation conditions and recovering a gaseous product from the carbonylation zone. The catalyst system includes a solid-phase component comprising platinum and tin deposited on an activated carbon support and a vaporous component comprising at least one of the halide promoters described above.
The carbon monoxide may be fed to the carbonylation zone either as purified carbon monoxide or as a mixture of hydrogen and carbon monoxide. Although hydrogen is not part of the reaction stoichiometry, hydrogen may be useful in maintaining optimal catalyst activity. The preferred ratio of carbon monoxide to hydrogen generally ranges from about 99:1 to about 2:1, but ranges with even higher hydrogen levels may be useful.
The present invention is illustrated in greater detail by the specific examples present below. It is to be understood that these examples are illustrative embodiments and are not intended to be limiting of the invention, but rather are to be construed broadly within the scope and content of the appended claims.